1. Field of the Invention
The present invention relates to a fiber laser oscillator that uses an optical fiber to oscillate and output a laser beam and a laser processing apparatus that applies a laser beam to a workpiece to perform a desired laser process.
2. Description of the Related Art
Recently, a fiber laser processing apparatus is coming into practical use, which applies a laser beam generated by a fiber laser oscillator to a workpiece to perform a desired laser process.
Generally, in a fiber laser oscillator used in a fiber laser processing apparatus, an optical fiber for oscillation having a core doped with a rare-earth element is optically disposed between a pair of optical oscillator mirrors; the core of the optical fiber is optically excited; an oscillating beam with a predetermined wavelength axially comes out from an end surface of the core and is reciprocated many times between the optical oscillator mirrors to be oscillated and amplified; and a coherent laser beam is taken out from one optical oscillator mirror (partially reflective mirror or output mirror). Normally, an optical lens is disposed between the fiber end surface and the optical oscillator mirror, and the oscillating beam reflected by the optical oscillator mirror is narrowed (converged) by the optical lens and returned to the core end surface of the oscillation optical fiber. The oscillating beam coming out from the core end surface of the oscillation optical fiber is collimated by the optical lens and becomes parallel light made incident on the optical oscillator mirror. To optically excite the core of the oscillation optical fiber, a laser diode (LD) is used for an excitation light source and the LD end surface excitation mode is employed to allow LD light (excitation light) to be condensed and made incident on the core end surface through the optical oscillator mirror and the optical lens.
As compared to a typical solid laser oscillator using a block-shaped crystal for an active medium, the particularities (drawbacks and advantages) of the fiber laser oscillator exist in that the core of the optical fiber is used as an active medium. With regard to the drawbacks of the fiber laser oscillator, it is problematic that the end surface of the oscillation fiber easily burns out. That is, when the reflected light or returned light from the optical oscillator mirror is made incident on the end surface of the core with a diameter on the order of 10 μm, the power density is extremely high. Therefore, since the base material of the core, i.e., quartz glass has a damage threshold lower than crystals such as YAG, the end surface of the core tends to be burned by the energy of the incident light. Therefore, the laser oscillation in the optical oscillator itself causes burnout of the core end surface. In the laser processing apparatus, the reflected light from a processing point of a workpiece may be propagated through a laser emitting unit and a laser transmission system in the opposite direction and made incident on the end surface of the oscillation optical fiber, and the core end surface may also be burned out by such external light. If the core end surface of the oscillation optical fiber is burned out, the oscillation power of the fiber laser is reduced and the quality of the laser process is deteriorated.